what is the reason the upper rad hose is more likely to burst?

that's where the hot coolant from the motor is flowing into the radiator... aka, it's hotter

As has been said it gets hotter since it is the discharge from the engine to the radiator. It also is on the "discharge" side of the waterpump while the lower is on the suction side. The problems seem to be blowing up the upper and, if you don't put the spring back in the lower when you replace the hose, actually collapsing the lower hose due to the suction of the water pump.